Film machinability refers to the rate at which a film may pass through packaging equipment without jamming. Film machinability may be tested by a “force over forming collar” test, wherein a film is pulled over a forming collar with force, and the force is recorded. The higher the force needed, the less machinable the film. Film machinability may also be tested by a “hot slip” test, wherein the back-seal bar of a packaging machine is heated up to 290° F. (116° C.), closed, and the film is pulled over the forming collar with force. Again, the more force that is necessary, the less machinable the film.
For heat-sealable films, other important properties are sealability and hot tack. Film sealability is usually measured in terms of a minimum sealing temperature (MST). The lower the minimum sealing temperature of the film, the broader the range of temperature that may be used to seal the film. “Hot Tack” is the strength of a heat seal immediately after sealing while still in a hot condition, i.e. before it has cooled down to ambient temperature and achieved its final strength.
Unmodified heat-sealable films typically have an inherently high coefficient of friction (COF) and film-to-film blocking properties. Therefore, slip additives and antiblocking particulates are traditionally added to the film structure to lower the COF and provide improved machinability to produce, for example, food packages.
In multi-layered film structures, slip additives are typically added to the core layer and permitted to migrate toward the skin layer. Such migration is dependent on time, temperature and pressure. Typical slip additives are polymers of fatty acid amides, such as erucamide or oleamide. Fatty acid amide slip systems, however, have reduced functionality when the multi-layer film is laminated to other non-slip containing films or substrates; as a result, the COF increases after lamination. Therefore, the production and functionality of fatty acid amide slip systems is limited.
Improved COF and slip functionality may be achieved using polydimethyl siloxane (“PDMS”) to the skin layer of a multi-layer film. Immediately upon winding a multi-layer film with one skin layer containing PDMS, the PDMS is transferred to the opposite side of the multi-layer film structure and it is lubricated. Therefore, it is relatively easy to obtain a multi-layer film with excellent slip performance on both sides when PDMS is incorporated into the skin layer of a multi-layer film. Multi-layer films containing an appropriately low concentration of PDMS also tend to perform well and maintain a low COF during lamination and when formed into a package on packaging machines.
In some instances, however, certain converting steps for a multi-layer film that employs a PDMS slip system on a multi-layer film surface is difficult. As noted above, the PDMS transfers from one surface of the multi-layer to the opposite surface upon winding of the film on a spool or roll. The surface having the PDMS transferred thereon becomes contaminated and consequently makes printing and ink adhesion more difficult. Additionally, if printing and laminating are done in two sequential steps, i.e., out-of-line, then PDMS may also transfer to the surface of the ink and cause future lamination bonding strengths to be low or inconsistent.
Therefore, there is a need for a heat-sealable multi-layer film structure having improved machinability using a slip system combined with improved print performance and improved laminations performance. The instant invention meets this and other needs.